I don't think there's a rule about not being able to finish a match you had the slot for when it started. I finished my match with RMG when he was the electric GT, even though the gym itself was gone by the end of the match.

Regardless, if you just don't feel like completing the match, I'd rather you forfeit. Because I see no reason why I should cancel.

I'm really mostly looking to cut TKF some slack on this one and let him get another gym match reffed. And are you sure there isn't like a 'let's get it over with' slot rule for matches in gyms that don't exist anymore?